1. Field of the Invention
The present invention relates to techniques for facilitating reliable medium access by multiple terminals in a wireless communication system or network.
2. Discussion of the Known Art
A common problem in wireless communication systems or networks, is controlling access to the medium in such a manner as to ensure reliable transfers of data among participating stations or nodes, Quality of Service (QoS), and fair access to the medium by all participants. A medium access controller (MAC) is normally employed to address these issues. Examples of wireless systems that employ MACs include various versions of WiFi (IEEE Standard 802.11), and various versions of WiMAX (IEEE Standard 802.16). All relevant portions of the mentioned IEEE standards are incorporated by reference.
Wireless access may be accomplished using either one of two methods, viz., reservation or contention. WiMAX is primarily a reservation based system wherein data is not transmitted unless one or more slots are reserved for the transmission. That is, in a reservation based system, the transmitting station (node) knows a priori that no other terminal will attempt to access the medium during certain reserved time slots. WiFi is primarily a contention based system where multiple terminals may attempt to access common medium resources (e.g., time, frequency, or code) simultaneously. Heuristics are normally employed to ensure that certain system objectives (fairness, QoS, etc.) for access are met. Unless reservations are preconfigured (not typical), some sort of contention access is required if for no other reason than to establish reservations for further transmissions. As such, virtually all wireless systems must incorporate some form of contention-based access control as part of their system.
Many forms of contention access are currently employed. For example, in ALOHA, wireless stations simply transmit as desired, each assuming that no other station is transmitting at the same time. If a second station is transmitting so that a first station's message is not properly received, a so-called collision has occurred. Thus, most ALOHA based systems also employ mechanisms to acknowledge receipt of a message, and retransmit messages that fail to reach their destination. The theoretical maximum efficiency for ALOHA is 18%. See, F. A. Tobagi, “Multiaccess Protocols in Packet Communication Systems”, IEEE Transactions on Communications, Vol. COM-28, No. 4, pages 468-88 (April 1980) (hereafter “Tobagi”), at page 471. All relevant portions of the cited Tobagi article are incorporated by reference. By introducing additional complexity and delay, higher efficiencies can be achieved. For example, slotted ALOHA (S-ALOHA) can increase the theoretical maximum efficiency of pure ALOHA to 36% by dividing the resources on the medium into time slices, thus ensuring that a transmission made within a given time slice will potentially interfere only with other transmissions that are made during the same time slice. Id.
Carrier sense multiple access (CSMA) is another form of contention access. CSMA enhances ALOHA by requiring the transmitting station initially to check if the desired medium resources are occupied. If not, the station transmits using the desired resources. CSMA has a maximum theoretical efficiency in excess of 90% depending on many medium properties. Tobagi, at page 473. The performance of CSMA degrades with increasing distance between the stations, however. That is, for a constant data packet size, as range delay (the propagation time between all stations attempting to access the medium) increases, performance of CSMA degrades. For a broadcast system with a repeater where all transmissions are synchronized to a single point in space and time (such as a satellite system), the point at which maximum theoretical performance of CSMA degrades to the maximum theoretical performance of S-ALOHA is roughly when the range delay is one third of the time the packet occupies the medium. Tobagi, at 473-74. Another key degradation occurs when there are “hidden” nodes. These are nodes that cannot hear other nodes that are actively transmitting in the network, so that the probability of a collision is increased. This also decreases the effectiveness of CSMA, and is a common problem, for example, in mobile ad hoc networks (MANET).
While CSMA has a higher efficiency than other contention mechanisms in many situations, it can sometimes be less effective than S-ALOHA because of the mentioned range and hidden node issues. In such environments, it may be advantageous to switch over to S-ALOHA. As far as is known, however, no method or apparatus capable of combining certain features of CSMA and S-ALOHA access so as to optimize the performance of a given network, has been disclosed or otherwise made available. A need therefore exists to provide such a capability.